NASA's Mars Atmosphere and Volatile Evolution spacecraft, called MAVEN, is currently orbiting the Planet studying its atmosphere and they've discovered that once the planet cooled and lost its global magnetic field, it turned into the cold and dry tundra we see today.

The MAVEN project's aim is, to determine how much of Mars’ dense atmosphere and water have been lost to space, the likelihood being that it was caused by solar wind and storms, and how these processes have changed the climate on the Red Planet over its history.